The present invention relates to medical lead systems and, more particularly, to insertion stylets used with medical stimulating leads.
The term “stylet,” as used in this disclosure, is an implement inserted into the lumen of a stimulating lead to stiffen the lead and to facilitate its insertion into a target tissue. The term “rod,” in one form of use, is an implement that is placed inside a cannula to provide support to the cannula while it is inserted into target tissue. The term “rod,” in another form of use, describes an elongate object.
The term “lead,” as used herein, will refer to an elongate conductor covered by insulation and which conductor is connected to an electrode contact. An “electrode contact” is a conductive material that is exposed to the tissue to be stimulated. An “electrode”, on the other hand, generally refers to that part of the lead that includes the electrode contact as well as a portion of insulation or other lead structure near the electrode contact. The term “electrode” may be used, herein, interchangeably with a stimulating lead. The term “tract” refers to an individual pathway formed in tissue, e.g., the brain, by inserting a microelectrode, a macroelectrode, a lead or an associated cannula into that tissue.
The term “microelectrode” refers to a recording electrode, for instance, used in deep brain stimulation (DBS), which microelectrode can be essentially an insulated conducting wire that has at least the distal portion of the wire uninsulated to receive electrical signals from the recorded tissue. The term “macroelectrode” will refer to a DBS temporary stimulating electrode having an electrode contact and parts connected to the electrode contact, which macroelectrode is intended as a temporary test electrode to perform macrostimulation. Macrostimulation involves stimulating many brain cells at once.
Deep brain stimulation is being increasingly accepted as a viable treatment modality. In particular DBS applied to the thalamus for treatment of tremor was approved by the FDA in 1997. Subsequently, other diseases, such as Parkinson's disease, dystonia and chronic pain, among others, have been identified as candidates for treatment with deep brain stimulation.
Implantation of a lead for deep brain stimulation generally involves the following preliminary steps: (a) anatomical mapping and (b) physiological mapping. Anatomical mapping involves mapping segments of an individual's brain anatomy using non-invasive imaging techniques, such as magnetic resonance imaging (MRI) and computed axial tomography (CAT) scans. Physiological mapping involves localizing the brain site to be stimulated. Step (b) can be further divided into: (i) preliminarily identifying a promising brain site by recording individual cell activity with a microelectrode and (ii) confirming physiological stimulation efficacy of that site by performing a test stimulation with a macroelectrode.
Microelectrode recording is generally performed with a small diameter electrode with a relatively small surface area optimal for recording cell activity. The microelectrode may be essentially a wire which has at least the distal portion uninsulated to receive electrical signals. The microelectrode functions as a probe to locate a promising brain site. Since a number of attempts may be required to locate the precise target site, it is desirable that the microelectrode be as small as possible to minimize trauma when the microelectrode is placed into the brain, in some cases, multiple times.
Once a brain site has been identified, a macroelectrode is used to test whether the delivered stimulation has the intended therapeutic effect. A macroelectrode is a temporary stimulating electrode which is not intended to be chronically implanted. Because macrostimulation involves stimulating many cells at once, an optimal electrode for macrostimulation requires a larger contact surface area compared to a microelectrode, which merely records electrical activity from a single cell or a few cells. For this reason, the electrode contact surface of a macroelectrode is generally larger than the electrode contact surface of a microelectrode. After a promising brain site has been identified with microelectrode cell recordings, the macroelectrode can be retraced through the same pathway to the same brain site.
Test stimulation with the temporary macroelectrode may need to be performed in a number of tracts in order to localize the site which provides the proper physiological effect. Because the macroelectrode may need to be repeatedly inserted into the brain, the macroelectrode must be durable, stiff and resistant to buckling. As the macroelectrode is intended for repeated use, it is made from a sterilizable material.
After macrostimulation has confirmed that stimulation at the brain site provides the intended physiological effect, the macroelectrode is withdrawn from the brain and a DBS lead is permanently implanted at the exact site.
Keeping in mind the above general steps, a conventional procedure for carrying out DBS may involve the following detailed steps: (1) place a stereotactic frame on the subject, which stereotactic frame is a device temporarily mounted on the head to assist in guiding the lead system into the brain; (2) perform MRI or equivalent imaging of the subject with the stereotactic frame; (3) identify a theoretical target using a planning software; (4) place the subject with the stereotactic frame in a head rest; (5) using scalp clips, cut the subject's skin flap in the head, exposing the working surface area of the cranium; (6) place the stereotactic arc with target coordinate settings and identify the location on skull for creation of a burr hole; (7) remove the arc and drill a burr hole in the patient's skull; (8) place the base of the lead anchor; and (9) with the microelectrode recording drive attached, and with an appropriate stereotactic frame adaptor inserted into the instrument guide, place the stereotactic arc.
Next, (10) advance a microelectrode cannula and an insertion rod into the brain until they are approximately 25 mm above the target; (11) remove the insertion rod, leaving the cannula in place; (12) insert a recording microelectrode such that the tip of the microelectrode is flush with the tip of the microelectrode cannula; (13) connect the connector pin of the recording microelectrode to a microelectrode recording system; (14) starting approximately 25 mm above the target, advance the microelectrode into a recording tract at the specified rate using the microdrive; and (15) if the target is identified, proceed to step 16. If the target is not identified, proceed with the following: (17) using the recording results and pre-operative imaging, (a) determine a new set of coordinates for the theoretical target; (b) disconnect the recording microelectrode from the microelectrode recording system; (c) remove the recording microelectrode cannula and recording microelectrode; and (d) adjust the coordinates of the stereotactic frame. Then, continue at step 10, above.
Next, (16) remove the recording microelectrode cannula and recording microelectrode; (17) insert a macroelectrode insertion cannula and rod until they are approximately 25 mm above the target; (18) remove the insertion rod, leaving the macroelectrode insertion cannula in place; (19) insert a stimulating macroelectrode, and advance it to the target stimulation site identified by the recording microelectrode; (20) using macrostimulation, simulate the stimulation of the chronic DBS lead to ensure proper physiological response; (21) remove the macroelectrode and cannula; (22) insert a DBS lead insertion cannula and an insertion rod, and advance both to approximately 25 mm above the stimulation site; (23) remove the insertion rod; (24) insert a DBS lead, with stylet, through the insertion cannula, and advance the lead/stylet to the stimulation site; (25) electrically connect the lead to a trial stimulator; and (26) perform the desired stimulation and measurements using any one or combination of four electrodes on the DBS lead.
Next, (27) if the results are favorable, proceed to step 28. If the results are not favorable, proceed with the following: (a) using the macrostimulation results, and microelectrode recording results, as well as pre-operative imaging, determine a new set of coordinates for the theoretical target; (b) remove the lead and stylet; (c) remove the insertion cannula; (d) adjust the coordinates of the stereotactic frame; and (e) continue at step 10, above.
Next, (28) remove the stylet, followed by the insertion cannula; (29) using macrostimulation, verify that micro-dislodgement of the DBS lead has not occurred; and, finally, (30) lock the DBS lead in the lead anchor. Some physicians might use additional steps, fewer steps, or perform the steps in a different order.
The above-described surgical procedure is far from ideal. There are many steps which lengthen the surgical procedure and increase surgical risk. For example, retracing the microelectrode or macroelectrode pathway increases the chance for misalignment and misplacement. In particular, misalignment may occur in retracing the location of the microelectrode with the macroelectrode and, further, in retracing the location of the macroelectrode with the permanent DBS lead. Retracing problems arise because separate cannulas must be placed each time into the brain to introduce the microelectrode, the macroelectrode and the permanent DBS lead. The use of three, separate cannulas significantly heightens the chance of mistracing and adds additional steps to the lead implant procedure. Mistracing can add surgical time because the procedure must be performed over and because expensive stimulating leads are likely to be scrapped. In addition, more procedural steps not only increases surgical time, but also increases the risk of peri-operative complications and the chance for post-operative infections.
In addition to DBS, another common medical application that uses a stylet in concert with a stimulating lead is spinal cord stimulation (SCS). Spinal cord stimulation is a well-accepted clinical method for reducing chronic, intractable pain. A spinal cord stimulation system typically includes an implanted pulse generator (IPG) and a stimulating lead, which lead is comprised of lead conductor wires and electrode contacts that are connected thereto. The IPG generates electrical pulses that are delivered to the dorsal column nerves within the spinal cord through the electrode contacts which are implanted along the dura of the spinal cord. In a typical situation, the attached leads exit the spinal cord and are tunneled around the torso of the patient to a subcutaneous pocket where the pulse generator is implanted. Representative SCS systems and leads are disclosed in the following patents: U.S. Pat. Nos. 3,646,940; 3,724,467; and 3,822,708.
Spinal cord stimulation requires a lead/stylet combination that has a very high resistance to kinks and high buckling strength since the lead/stylet combination is pushed through muscle, fascia and other tissue. The lead should preferably have a small diameter profile to facilitate ease of insertion into tissue. Because the lead has a small diameter, the stylet, by necessity, must also have a small diameter profile in order to fit inside the lead lumen. At the same time, the lead/stylet should preferably offer mechanical characteristics which enable multiple insertions into tissue, without breaking or permanently bending.
Conventional stylets and guidewires for the SCS application are typically made of stainless steel or tungsten. Tungsten is a malleable, linear elastic material. A stylet made from tungsten is flexible and does not easily break but, unfortunately, has poor kink resistance. “Kink resistance” will be used, hereinafter, as a term to describe the ability of the wire to be bent into a relatively tight bend radius without incurring permanent deformation. Once a stylet kinks, the stylet/lead combination may have to be withdrawn from the tissue because the permanent bend in the stylet makes it difficult to steer the lead/stylet combination within the tissue. The ability to steer the lead is critical for achieving optimal stimulation in spinal cord stimulation where a positional difference of a few millimeters may mean the difference between poor or effective stimulation. If the stylet kinks during use, both the stylet and lead may need to be scrapped because the bent stylet cannot be easily extracted from the lead without causing further damage to the lead or dislodging the lead from the tissue site.
Super-elastic materials provide excellent kink resistance but have poor resistance to buckling forces and torquing and so a stylet made from this material alone would not be suitable for use with a spinal cord stimulation lead. Stylets and guidewires manufactured from combinations of linear and super-elastic materials have been evaluated, as disclosed in U.S. Pat. Nos. 6,214,016; 6,168,571; 5,238,004; 6,270,496 and 5,957,966. A guidewire, for example, is taught in U.S. Pat. No. 6,214,016. This guidewire, however, is very complicated to assemble and use and has a relatively large diameter.
Accordingly, there is a need to have a stylet or guidewire that has a very small diameter and exhibits mechanical properties ideal for SCS or DBS application, including high buckling strength and resistance to kinking, yet is relatively inexpensive and easy to manufacture. Such an improved stylet would allow multiple insertions of the lead/stylet into tissue and reduce the scrapping of leads and stylets. For the DBS application, there is a further need to reduce the number of surgical steps in order to reduce surgical time and operative risk and to improve the accuracy of placing the permanent DBS lead.